The production of synthetic fibres having a modified fibre cross section according to melt- and wet-spinning technology has been known for many years. Thus, for example, polyamide and polyester fibres are preferably produced from profiled spinnerets by melt-spinning in order to achieve special effects with respect to gloss, hand, lustre and final quality of the fabric. The effects which the change in the filament cross-sectional form of synthetic fibres has in particular on the final quality and on the behaviour of finished goods emerges, for example, from the reports by F. Bolland in Chemiefasern 13 (1963), pages 42-45 and 106-109, and from the article by H. Bieser and R. Hesse in Chemiefasern 17 (1967), pages 262-268. H. Knopp reports in Lenzinger Berichten 36 (1974), pages 160-167, on improved utility characteristics and stylish effects in the production of nylon-6 profiled yarns with a triangular profile. A Lehnen and G. Satlow report in Chemiefasern und Textilindustrie, March 1975, pages 251-254, on the improved soiling behaviour of textile flooring materials made of polyamide-6,6 yarns having a deeply-lobed trilobal cross-section. In addition to melt-spinning technology, synthetic fibres which are modified in cross section, for example cross-section-modified acrylic fibres, may also be produced by wet-spinning. Thus, acrylic fibres having a triangular fibre cross section are available on the market, and are distinguished by a high colour brilliance.
Many attempts have also been made hitherto to produce profiled acrylic fibres from a spinning solution by dry-spinning. Thus, for example, the profile spinning of polyacrylonitrile by dry-spinning is supposed to be known according to U.S. Pat. No. 3,760,053, but no experimental evidence is disclosed. Furthermore, U.S. Pat. No. 3,340,571 also names acrylonitrile homo- and copolymers among the dry-spinnable polymers which produce profiled fibres using profiled nozzles, but experiments are actually submitted only for cellulose acetate. In any case, no commercial process has been disclosed hitherto for the production of cross-section-modified acrylic fibres by dry-spinning. For example, when dry-spinning polyacrylonitrile spinning solutions in a conventional concentration and using profiled nozzles, a dumbbell-shaped cross section is only ever obtained, i.e. for example, using an acrylonitrile copolymer, consisting of 93.6% by weight of acrylonitrile, 5.7% by weight of methyl acrylate and 0.7% by weight of sodium methallyl sulphonate and having a K value of 81, and a 32% by weight spinning solution in dimethylformamide. If it is attempted further to increase the solids content, then spinning solutions of this type gelatinize upon cooling even at temperatures of from about 50.degree. to 80.degree. C., so that trouble-free spinning is impossible.